Recent clinical and experimental observations have converged to suggest that plasmapheresis (plasma exchange) might be of benefit in the treatment of acute Guillain Barre Syndrome (GBS). Several institutions have reported mixed, but generally promising, preliminary results of the effect of plasmapheresis in GBS. However, the monophasic natural history of acute GBS requires that a controlled clinical trial be undertaken to determine whether this treatment is in fact beneficial. Therapy which could shorten the duration of this paralytic disease or lessen residual disability would be welcomed for a disorder which affects an estimated 1500 Americans annually and is a world-wide problem. However, if it has little or no benefit, the potential risk and considerable expense of plasma exchange would preclude its use for GBS. This issue is timely, since without a controlled trial use of plasmapheresis in GBS is likely to become widely disseminated. With this rationale, 20 academic medical centers have developed this proposal to undertake a controlled trial of plasma exchange versus best available supportive medical care. 250 patients will be randomized into the study, over 24 months. Patient status will be measured by simple and reproducible clinical grade scores and improvement analyzed by duration in grade and other measures. Patient outcome in both study arms will be compared at intervals to 6 months after randomization.